John Keats' poem "When I Have Fears" is a poem that touches on multiple themes, one of them being Death. Two other themes that stand out in the poem are Life (contrasting Death) and Fear.
An initial reading of this English poem certainly leads the reader's mind into thoughts of death. This is evident in the opening line:
When I have fears that I may cease to be
The reader understands that Keats is talking about the cessation of life and the end of experiences in the physical realm.
However, a deeper reading of the poem brings to understanding the fact that Keats is also celebrating all that is wonderful in life, even as he alludes to death. He talks of the joy of writing, and the joy of reading books from an abundant library.
Keats also talks of love. It is evident he is talking about his love for a fine woman, when he writes:
And when I feel, fair creature of an hour,
Consequently, when Keats realizes that death will end his creative literary urges and his romantic feelings for his love interest, he begins to experience fear. He does not want these experiences to end, or at least end before he's indulged himself completely, and for a long period of time, in these life pleasures.
Therefore, this poem is about Death, but also about all that is exciting and desirous in Life. The poem alludes to human beings fearing they will not get enough living in before they die.